Using your SLR camera in manual mode, you can measure light level. Set camera ASA speed to 200. Set camera shutter speed to 1/125. Hold a large, white card (or sheet of paper) in the proposed plant location so that it gets max illumination. Be sure that nothing but the white card is showing in the viewfinder. Adjust f-stop on camera (or lens) until camera meter reads “correct exposure”. f4 = 64 foot-candles, f5.6 = 125 foot-candles, f8 = 250 foot-candles, f11 = 500 foot-candles, f16 = 1000 foot-candles, f22 = 2000 foot-candles, f32 = 4000 foot-candles.
I use this technique to: Determine “hot” spots in the room, I put my youngest plants there. Determine “weak” spots – install reflectors as needed to bring up light levels. Determine when bulbs are getting weak, instead of “it looks dimmer than before”.